1. Technical Field of the Invention
This invention relates to a constant current loop measuring system for generating measurable voltages across a resistor disposed in an environment and a reference resistor and determining a difference between these voltages to calculate a change in the voltage across the resistor disposed in the environment, this change representing a predetermined condition of the environment.
2. Description of the Prior Art
When performing scientific experiments, it is imperative that measurements taken during these experiments are as accurate as possible. When measuring a characteristic or a change in a characteristic of an environment, it is common to dispose in the environment a resistor whose resistance varies in proportion with the characteristic of the environment being measured.
It is common to use such a resistor as part of a Wheatstone bridge system. A Wheatstone bridge is a two-branched voltage divider network usually consisting of three fixed resistors and a variable resistor. The variable resistor is disposed in the environment to measure the predetermined characteristic. A current is passed through the resistors and a voltage difference or current difference appearing at the output of the Wheatstone bridge is measured. This voltage or current difference is proportional to the change in the variable resistor due to the predetermined characteristic. The magnitude of the predetermined characteristic is then calculated based on this voltage or current change.
A problem that exists in using a Wheatstone bridge is the existence of parasitic resistances throughout the Wheatstone bridge system. These parasitic resistances are due to the connecting wires or additional components such as slip rings, etc., which couple the resistors together. The parasitic resistances generate parasitic voltages when current is passed through the resistors in the Wheatstone bridge. These parasitic voltages can cause erroneous voltage or current differences to appear at the output of the Wheatstone bridge and therefore cause inaccurate measurements. Furthermore, these parasitic resistances may vary due to thermal, mechanical, chemical or other conditions of the environment and thus, it is extremely difficult to provide circuitry to compensate for these parasitic resistances.
Examples of circuits known in the art which attempt to eliminate the effect of parasitic resistances are four-wire Kelvin circuits or a circuit which connects three wires to the remote variable resistor disposed in the environment. This three wire circuit attempts to electrically subtract the parasitic resistance variations in each of the current carrying leads connected to the variable resistor. The subtraction is effected by connecting the leads to adjacent arms of the Wheatstone bridge so that the parasitic resistances effectively cancel each other at the output of the Wheatstone bridge.
This approach is effective in moderate temperature environments but causes the measurement system to be less sensitive due to the increase in circuit resistance caused by the lead wires. Also, because the wires and associated components are not identical, in severe temperature environments, the parasitic resistances vary. This results in an unreliable output and inaccurate measurements.
Another method for reducing the effect of parasitic resistances in the Wheatstone bridge or Kelvin circuit is to pass a constant current through the Wheatstone bridge or Kelvin circuit. This approach satisfactorily reduces the parasitic effect of the wiring connecting the constant current source to the Wheatstone bridge or Kelvin circuit. But, this approach does not reduce the effects of the parasitic voltages caused by the parasitic resistances within the Wheatstone bridge or Kelvin circuit.
There are other problems that occur when using Wheatstone bridge circuits. One is that the variations in voltage output are very small in comparison to the voltage drop across the arms of the Wheatstone bridge circuit. That is, for example, the voltage across each arm of the Wheatstone bridge can be several volts while the variation voltage due to the resistance change caused by the predetermined characteristic in the environment is usually on the order of several millivolts. Another is that the electrical output (either voltage or current) of the Wheatstone bridge circuit is always a nonlinear function of the resistance change of the variable resistor. This non-linearity characteristic causes the data processing necessary for determining the value of the environmental characteristic in proportion with this voltage or current variation to be more complex. Furthermore, the output of a Wheatstone bridge circuit is a function of the internal resistances of the circuit and not a function of the change in the variable resistor alone. This makes calibration more difficult.
It is therefore necessary and desireable to develop a measurement system having a reliable output that is unaffected by parasitic resistances that may exist in the system. Furthermore, it is also beneficial that the output of the system be linear thus simplifying the data processing necessary to calculate the change in environmental characteristic based on the change in output voltage or current.